Which of napoleon's campaigns represent the furthest extent of his victories and a major turning point in his military career?

It would be the Russian campaign of Napoleon. The Russian campaign is the biggest failure and disaster Napoleon ever experience in his entire career in which other nation sees him as unstoppable and formidable adversary soon come to a halt in this campaign. His Grand Army was destroyed by the Russian winter and was forced to retreat back to his empire. Such destruction in his army leave him vulnerable and weak to fend off the coalition army that starts to renew their attacks after the failed invasion.

B) hypothesis. its guessing and your guess could be changed quickly based on the information.